Precious tropical woods are preferred over native woods for demanding wooden structures in interior and exterior areas because they have advantageous properties. On the other hand, the increased use of precious tropical woods leads to the decimation of tropical rainforests and should therefore be rejected for ecological reasons. Moreover, sufficient amounts of local hardwoods and softwoods are available, which, however, often do not meet the technical demands.
This dilemma gave rise to the idea of thermally treating low-grade woods to improve their properties. According to EP 0 759 137 A1 and EP 0 695 408 A1 methods are for example described for modifying woods by the application of elevated temperatures in such a manner that they cover a profile of requirements which can usually only be achieved by precious tropical woods. In particular their resistance towards mildew and rot is improved. A disadvantage is that the treatment has to take place at high temperatures of more than 200° C. and that some of the resulting properties cannot be improved to the desired degree.
As an alternative methods are known in which wood is modified by impregnation with reactive monomers and the subsequent cross-linking of the same. Such methods are known for example from WO 2006/117 159 A1 and WO 2004/033 171 A1. Wooden materials are formed which have a high degree of hardness, weatherproofness and dimensional stability. However, a disadvantage is that large amounts of impregnating agents have to be used which additionally considerably increase the specific weight of the wooden material. Furthermore, the formaldehyde content of the impregnating agents that are used results in undesired emissions from the wooden material.
The treatment of cellulose-containing materials with cyanamide is basically known. Thus, as described in U.S. Pat. No. 3,051,698 and U.S. Pat. No. 3,380,799, cellulose is treated with cyanamide at pH values above 8.5 and subsequently acidified which results in cationically modified celluloses with ionic properties which are for example suitable as adsorption agents.
When cellulose fibres are reacted at room temperature with cyanamide, reactive aldehydes and amines according to DE 16 19 047 A1 polymer-modified celluloses are formed which are particularly suitable for electro-insulating papers.
However, no method for treating cellulose-containing materials and in particular wood with cyanamide is known in which the addition of further reactive substances can be dispensed with.